Convert A System Array To A List In C#

Introduction

In C#, a System Array is an abstract class that can only be instantiated through a function. From the class name itself, it's hard to tell what type it encapsulates so you'll need to plan and be sure what time it is ahead of time. Since System Array is not in the collection namespace but it is still considered a collection since it implements IList. There are other types of arrays in the collection namespace so I will be referring to this array as a system array to avoid confusion. With this, we can convert System Array to a List so I'll go over the methods one by one and provide an analysis of each one.

What Methods Convert a System Array into a List?

There are a few ways to do this and all of them involve a cast of some sort. One is to write our method to loop the system array and add them to a list. The second uses an array cast and then uses ToList. Lastly, the other method is to use OfType to cast the objects and then use ToList to assign them to a list. All of these methods can get the result that we want but the difference in performance will be key to determining which is the best to use.

Loop Method

The first method to convert all the System Array items to a list is through the brute force method of looping through all the items. Then apply a cast to each item retrieved and then adding it to the list. Examine the below code.

Array systemArray = Array.CreateInstance(typeof(int), 4);//Create System Array with type int and length of 4
systemArray.SetValue(10, 0);//Set value to 10 at index 0
systemArray.SetValue(21, 1);//Set value to 21 at index 1
systemArray.SetValue(32, 2);//Set value to 32 at index 2
systemArray.SetValue(43, 3);//Set value to 43 at index 3
List<int> list = new List<int>();
for (int i = systemArray.GetLowerBound(0); i <= systemArray.GetUpperBound(0); i++)
{
    list.Add((int)systemArray.GetValue(i));//Get value from System Array then cast it to int then add it to the list.
}

Console.WriteLine("list:" + string.Join(",", list));//Print to screen

Code Output

list:10,21,32,43

This is the most basic way of converting the two types but let's also test the performance of this.

Loop Method Speed Test

In this test, I will run 10 tests which will have 100 function calls of the loop method. The loop method will run on 1 million objects. This will be the baseline test for all the tests.

using System.Diagnostics;
int numberOfTests = 10;//Number of tests 
List<double> testSpeedList = new List<double>();

for (int i = 0; i < numberOfTests; i++)
{
    testSpeedList.Add(GetConvertArrayToListByLoopSpeedTest());
}
Console.WriteLine($"Array To List By Loop Method Average speed:{Math.Round(testSpeedList.Average())}ms, In {numberOfTests} tests");

double GetConvertArrayToListByLoopSpeedTest()
{
    int numberOfFunctionCalls = 100;//Number of function calls made
    Stopwatch stopwatch = new Stopwatch();
    Array array = GetArrayData();//Get intial random generated list
    for (int i = 0; i < numberOfFunctionCalls; i++)
    {
        stopwatch.Start();//Start the Stopwatch timer
        List<int> convertedList = ConvertSystemArrayToListByLoop(array);//Convert System Array to List
        stopwatch.Stop();//Stop the Stopwatch timer
    }
    stopwatch.Stop();//Stop the Stopwatch timer
    Console.WriteLine($"Function calls:{numberOfFunctionCalls}, In {stopwatch.Elapsed.Minutes}m {stopwatch.Elapsed.Seconds}s {stopwatch.Elapsed.Milliseconds}ms");
    return stopwatch.Elapsed.TotalMilliseconds;
}

List<int> ConvertSystemArrayToListByLoop(Array systemArray)
{
    List<int> list = new List<int>();
    for (int i = systemArray.GetLowerBound(0); i <= systemArray.GetUpperBound(0); i++)
    {
        list.Add((int)systemArray.GetValue(i));//Get value from System Array then cast it to int then add it to the list.
    }
    return list;
}

Array GetArrayData()
{
    Array array = Array.CreateInstance(typeof(int), 1000000);
    for (int i = array.GetLowerBound(0); i <= array.GetUpperBound(0); i++)
    {
        array.SetValue(GetRandomInt(0, 10000), i);
    }
    return array;
}

int GetRandomInt(int minNumber, int maxNumber)
{
    Random random = new Random();//Create Random class
    int randomInt = random.Next(minNumber, maxNumber);//Get a random number between 1 and the maxnumber
    return randomInt;
}

Code Output

Function calls:100, In 0m 3s 705ms
Function calls:100, In 0m 3s 582ms
Function calls:100, In 0m 3s 576ms
Function calls:100, In 0m 3s 458ms
Function calls:100, In 0m 3s 552ms
Function calls:100, In 0m 3s 402ms
Function calls:100, In 0m 3s 483ms
Function calls:100, In 0m 3s 568ms
Function calls:100, In 0m 3s 545ms
Function calls:100, In 0m 3s 445ms
Array To List By Loop Method Average speed:3532ms, In 10 tests

From this basic loop, the average speed from the test is about 3 and a half seconds. There are other ways that C# provides for us to handle this use case.

Cast and ToList Method

Another way we can solve this issue is to cast The System Array object to an array. Then convert that array to a list.

Array systemArray = Array.CreateInstance(typeof(int), 4);//Create System Array with type int and length of 4
systemArray.SetValue(10, 0);//Set value to 10 at index 0
systemArray.SetValue(21, 1);//Set value to 21 at index 1
systemArray.SetValue(32, 2);//Set value to 32 at index 2
systemArray.SetValue(43, 3);//Set value to 43 at index 3
List<int> myList = ((int[])systemArray).ToList();
Console.WriteLine("myList:" + String.Join(",", myList));//Print to screen

Code Output

myList:10,21,32,43

This gives us the same output as before so this cast-to-array succeeds. Let's check out the performance of this method.

Cast and ToList Method Speed Test

This will be the same test as before of 10 tests and 100 function calls for 1 million objects in the system array.

using System.Diagnostics;
int numberOfTests = 10;//Number of tests 
List<double> testSpeedList = new List<double>();

for (int i = 0; i < numberOfTests; i++)
{
    testSpeedList.Add(GetConvertArrayToListByToListSpeedTest());
}
Console.WriteLine($"Convert System Array To List By Cast and ToList Method Average speed:{Math.Round(testSpeedList.Average())}ms, In {numberOfTests} tests");

double GetConvertArrayToListByToListSpeedTest()
{
    int numberOfFunctionCalls = 100;//Number of function calls made
    Stopwatch stopwatch = new Stopwatch();
    Array array = GetArrayData();//Get intial random generated list
    for (int i = 0; i < numberOfFunctionCalls; i++)
    {
        stopwatch.Start();//Start the Stopwatch timer
        List<int> convertedList = ConvertArrayToListByToList(array);//Convert Array to List
        stopwatch.Stop();//Stop the Stopwatch timer
    }
    stopwatch.Stop();//Stop the Stopwatch timer
    Console.WriteLine($"Function calls:{numberOfFunctionCalls}, In {stopwatch.Elapsed.Minutes}m {stopwatch.Elapsed.Seconds}s {stopwatch.Elapsed.Milliseconds}ms");
    return stopwatch.Elapsed.TotalMilliseconds;
}

List<int> ConvertArrayToListByToList(Array systemArray)
{
    List<int> list = ((int[])systemArray).ToList();//Cast it to int [] then Tolist.
    return list;
}

Array GetArrayData()
{
    Array array = Array.CreateInstance(typeof(int), 1000000);
    for (int i = array.GetLowerBound(0); i <= array.GetUpperBound(0); i++)
    {
        array.SetValue(GetRandomInt(0, 10000), i);
    }
    return array;
}

int GetRandomInt(int minNumber, int maxNumber)
{
    Random random = new Random();//Create Random class
    int randomInt = random.Next(minNumber, maxNumber);//Get a random number between 1 and the maxnumber
    return randomInt;
}

Code Output

Function calls:100, In 0m 0s 191ms
Function calls:100, In 0m 0s 75ms
Function calls:100, In 0m 0s 124ms
Function calls:100, In 0m 0s 130ms
Function calls:100, In 0m 0s 109ms
Function calls:100, In 0m 0s 87ms
Function calls:100, In 0m 0s 107ms
Function calls:100, In 0m 0s 62ms
Function calls:100, In 0m 0s 53ms
Function calls:100, In 0m 0s 148ms
Convert System Array To List By Cast and ToList Method Average speed:109ms, In 10 tests

This method is very fast at only 109 milliseconds and is so much faster than just looping through all the records.

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OfType and ToList Method

Lastly, we'll look at the OfType Method. This method filters out objects that can be cast as the type and returns an IEnumerable. We can use this method to convert a System Array to an IEnumerable and then to a list.

Array systemArray = Array.CreateInstance(typeof(int), 4);//Create System Array with type int and length of 4
systemArray.SetValue(10, 0);//Set value to 10 at index 0
systemArray.SetValue(21, 1);//Set value to 21 at index 1
systemArray.SetValue(32, 2);//Set value to 32 at index 2
systemArray.SetValue(43, 3);//Set value to 43 at index 3
List<int> myList = systemArray.OfType<int>().ToList();//Filter out all objects that can be cast to int then change the IEnumerable type to List with ToList
Console.WriteLine("myList:" + String.Join(",", myList));//Print to screen=

Code Output

myList:10,21,32,43

As expected this produces the expected result. Now let's take a look at the speed of this method.

OfType and ToList Method Speed Test

We'll test this in the same way as the other methods and let's see how it compares.

using System.Diagnostics;
int numberOfTests = 10;//Number of tests 
List<double> testSpeedList = new List<double>();

for (int i = 0; i < numberOfTests; i++)
{
    testSpeedList.Add(GetConvertArrayToListByTYpeOfAndToListSpeedTest());
}
Console.WriteLine($"Convert Array To List By OfType Then ToList Method Average speed:{Math.Round(testSpeedList.Average())}ms, In {numberOfTests} tests");

double GetConvertArrayToListByTYpeOfAndToListSpeedTest()
{
    int numberOfFunctionCalls = 100;//Number of function calls made
    Stopwatch stopwatch = new Stopwatch();
    Array array = GetArrayData();//Get intial random generated list
    for (int i = 0; i < numberOfFunctionCalls; i++)
    {
        stopwatch.Start();//Start the Stopwatch timer
        List<int> convertedList = ConvertArrayToListByTypeOfAndToList(array);//Convert Array to List
        stopwatch.Stop();//Stop the Stopwatch timer
    }
    stopwatch.Stop();//Stop the Stopwatch timer
    Console.WriteLine($"Function calls:{numberOfFunctionCalls}, In {stopwatch.Elapsed.Minutes}m {stopwatch.Elapsed.Seconds}s {stopwatch.Elapsed.Milliseconds}ms");
    return stopwatch.Elapsed.TotalMilliseconds;
}

List<int> ConvertArrayToListByTypeOfAndToList(Array systemArray)
{
    List<int> list = systemArray.OfType<int>().ToList();//Filter out all objects that can be cast to int then change the IEnumerable type to List with ToList
    return list;
}

Array GetArrayData()
{
    Array array = Array.CreateInstance(typeof(int), 1000000);
    for (int i = array.GetLowerBound(0); i <= array.GetUpperBound(0); i++)
    {
        array.SetValue(GetRandomInt(0, 10000), i);
    }
    return array;
}

int GetRandomInt(int minNumber, int maxNumber)
{
    Random random = new Random();//Create Random class
    int randomInt = random.Next(minNumber, maxNumber);//Get a random number between 1 and the maxnumber
    return randomInt;
}

Code Output

Function calls:100, In 0m 3s 841ms
Function calls:100, In 0m 3s 672ms
Function calls:100, In 0m 3s 792ms
Function calls:100, In 0m 3s 644ms
Function calls:100, In 0m 3s 781ms
Function calls:100, In 0m 3s 781ms
Function calls:100, In 0m 3s 791ms
Function calls:100, In 0m 3s 610ms
Function calls:100, In 0m 3s 760ms
Function calls:100, In 0m 3s 683ms
Convert Array To List By OfType Then ToList Method Average speed:3736ms, In 10 tests

This ends up being the slowest method and even slower than just looping through the list ourselves.

Conclusion

The best method for converting a system array to a list is to cast it to an array of that type and then use ToList. It has a really good performance that is far better than the other two methods. This call all be done on one line so it's compact and readable. Looping through the array and adding to a list is almost as slow as using OfType it isn't worth it to write unless you need to. Even though the type and ToList can fit one line of code, it is too slow and shouldn't be used compared to the other methods.

Know any other methods of doing this? Let me know in the comments.